1. Field of the Invention.
The present invention relates to fuel ignition systems of the direct ignition type, and more particularly, to a control circuit for use in such systems for controlling the operation of a fuel supply valve and spark generator of the system.
2. Description of the Prior Art.
In direct ignition type fuel ignition systems, a fuel supply valve is energized tentatively at the start of each heating cycle to supply fuel to a burner for ignition by sparks provided by a suitable spark generator. If the fuel fails to be ignited within a predetermined time, commonly referred to as a trial for ignition period, the valve is deenergized, and the system is locked out.
Direct ignition systems include a valve control circuit which includes a timing device, or more commonly an electronic timing circuit, which determines the duration of the trial for ignition interval, and effects the deenergization of the fuel supply valve whenever the fuel fails to be ignited before the end of the trial for ignition. One such system is disclosed in my U.S. Pat. No. 3,938,937 which was issued on Feb. 17, 1976. The patented system employs a capacitive timing circuit which permits a controlled switching device to be enabled to effect energization of a fuel supply valve during the trial for ignition interval which is defined by the charging time of a capacitor of the timing circuit.
If the fuel is not ignited before the capacitor is fully charged, the timing circuit disables the controlled switching device, causing the fuel supply valve to be deenergized. On the other hand, if the fuel is ignited during the trial for ignition period, flame current is supplied to the controlled switching device, overriding the timing capacitor to permit the fuel supply valve to remain operated for the duration of the heating cycle.
For the purpose of resetting the timing capacitor to its initial condition at the end of each heating cycle, a discharge network is provided for permitting the capacitor to discharge when the system is deactivated. The discharge network includes a plurality of resistors serially connected in parallel with the capacitor, providing a shunt discharge path of relatively high resistance for the capacitor. Thus, in the event of a power interruption, which is of a duration sufficient to cause the timing capacitor to discharge, the system may become locked out when power is restored.
Thus, it would be desirable to have a control arrangement which effects rapid discharge of such timing capacitor whenever the system is deactivated, but which allows the timing capacitor to provide its timeout function when the system is activated.
Another direct ignition system employing a capacitive timing circuit is disclosed in the U.S. Pat. No. 3,669,097 to Clay et al. which was issued on Nov. 9, 1971. In this system, the timing circuit includes a pair of timing capacitors which are charged substantially instantaneously upon application of power to the system. The timing capacitors slowly discharge over a discharge network, defining a trial for ignition interval by the time it takes one of the capacitors to discharge. The timing capacitors form a voltage divider network which establishes an operating potential for a controlled switching device, embodied as a field effect transistor, which when enabled, effects the energization of a fuel supply valve. When the capacitors are discharged, the field effect transistor is disabled, causing the valve to be deenergized. If a flame is established before the end of the trial for ignition interval, a flame sensing means, which is directly connected to the timing circuit, supplies flame current to the timing capacitors to maintain the capacitors charged. Thus, in this system, where timing capacitors are charged rapidly and then discharged to define the trial for ignition interval, the discharge network is used in determining the trial for ignition interval. Also, since the flame sensing means is directly connected to the timing circuit, under certain conditions, there may be undesirable interaction between the flame sensing means and the timing circuit in the absence of a flame.
A further consideration is that in direct ignition systems, the flame sensing means responds to the relatively large main burner flame to effect its control operation to maintain the system operating when a flame is established. In many instances, this requires the use of flame sensing probes of different lengths in similar control circuits, and careful positioning of the probe relative to the burner in order to prevent too large a flame from effecting reliable operation of the control circuit or causing the system to become locked out.